The present invention relates to differential gear mechanisms, and more particularly, to such mechanisms of the type commonly referred to as "locking differentials".
Differential gear mechanisms of the general type to which the present invention relates are broadly referred to as "limited slip differentials", and typically include a clutch pack which is operable to limit or retard differentiating action between the output gears (sidegears). More specifically, however, the present invention is intended for use on limited slips of the type referred to as "locking differentials", and will be described in connection therewith. In a locking differential, means are provided for engaging or locking the clutch set, rather than permitting it to slip (i.e., the slip is "limited" to the point of being nonexistent), thus substantially eliminating differentiating action between the output gears.
Locking differentials are produced and sold commercially by the assignee of the present invention, with the construction and operation of such locking differentials being illustrated and described in great detail in U.S. Pat. No. 4,389,909, assigned to the assignee of the present invention and incorporated herein by reference. In the device of the above-incorporated patent, there is a flyweight mechanism which rotates about its axis of rotation at a speed representative of the differential speed between the two output gears. Above a predetermined rotational speed (speed of differentiation), the flyweight mechanism is actuated to cause ramping of a cam member relative to another cam member. This ramping action of the cam members results in axial movement of the one cam member, which initiates engagement of an adjacent clutch pack. Typically, the clutch pack is of the "self-actuating" type, i.e., a small amount of frictional engagement within the clutch pack further retards rotation of the adjacent cam member, resulting in further ramping action, which, in turn, causes further axial movement of the cam member and further engagement of the clutch pack.
In the device of the above-incorporated patent, the clutch pack immediately adjacent the cam arrangement includes two types of clutch discs:
1. "Active" discs are those discs disposed immediately adjacent, and operably associated with, the axially movable cam member and which determine the rate of engagement of the overall clutch pack. PA1 2. "Inactive" discs are those discs which are not operably associated with the cam member, but provide at least a major portion of the torque capacity of the clutch pack.
One of the key performance criteria of a locking differential is the "time of engagement" of the mechanism, which is the period of time (.DELTA.T) required for the clutch pack to go from a predetermined, lower torque transmitting capacity to a predetermined, higher torque transmitting capacity. By way of example only, in connection with the development of the present invention, the time of engagement was based upon the .DELTA.T from 100 foot-pounds of torque capacity to 1000 foot-pounds of torque capacity. As is well known to those skilled in the art, one of the factors in determining the time of engagement is the cam angle (i.e., the angle defined between the cam surface and the transverse plane). In general, for a steeper cam angle, more force must be exerted to move the cam member axially, but it is relatively easier to unlock the cam (which occurs as a result of a torque reversal, or going from a right turn to a left turn, and vice-versa). On the other hand, for a shallower angle, relatively more force is transmitted by the cam member to the clutch, providing a tighter engagement of the clutch, and it is relatively harder to unlock the cam members.
In prior art locking differentials, the clutch packs have typically comprised a plurality of stamped steel clutch discs, with the discs typically including some sort of pattern stamped into the disc face to enhance frictional engagement between adjacent disc surfaces. Although the prior art locking differentials described above, utilizing conventional steel discs, have been generally satisfactory in operation, one particular operating situation has resulted in a performance problem. If a vehicle including a locking differential is in mud or snow, and is "stuck", the vehicle operator will typically try to "rock" the vehicle by putting the transmission, alternately, in forward and then reverse. Each time this is done, if the traction is different at the two drive wheels, a spin-out condition will occur, resulting in an engagement of the locking differential. Whenever a high number of engagements occurs within a fairly short time, the temperature of the device, and especially of the torque-transmitting portion of the differential (i.e, the clutch packs) increases substantially.
As is well known to those skilled in the art, with steel clutch discs, increasing temperature results in a decreased time of engagement (i.e., a longer .DELTA.T before engagement occurs). This decreasing time of engagement occurs because the coefficient of friction of the steel discs decreases with increasing temperature. The decreasing coefficient of friction results in a decreased overall torque capacity of the differential. In other words, with increased usage within a short period of time, the performance of the locking differential may degrade.